1. Field
The techniques and devices described herein relate generally to integration of optical data communication technology and electronic circuits, and relate particularly to an optical modulator driver circuit which is suitable for use with integrated silicon photonic interconnect technology and is compatible with a standard CMOS process.
2. Discussion of the Related Art
Many electronic devices use one or more integrated circuits to receive, store, process, and/or send data. An integrated circuit (“IC” or “chip”) may include a wafer of semiconductor material, such as silicon, on which one or more electronic circuits have been fabricated by applying a sequence of processing steps to the semiconductor wafer. These processing steps may include, for example, photolithographic patterning, material deposition, doping, annealing, material removal, and cleaning. For reasons that are understood by one of ordinary skill in the art (e.g., power dissipation, scalability, and/or cost of manufacturing), many ICs are fabricated using a standard CMOS (complementary metal-oxide semiconductor) manufacturing process, rather than a customized CMOS process or a non-CMOS process.
An electronic device may use data communication technology to move data from one location to another within the device, or to exchange data with another device. A variety of data communication technologies are known, including electrical and optical technologies. Electrical data communication technologies transport data by propagating electrical signals through metal interconnects (e.g., wires). Optical data communication technologies transport data by propagating optical signals (e.g., light) through optical interconnects (e.g., waveguides).
In an optical data communication system, data may be encoded in an optical signal by modulating one or more of the signal's properties, such as its phase, amplitude, frequency, or polarization. Such modulation may be achieved by changing an optical property of the waveguide through which the optical signal propagates, such as the waveguide's absorption coefficient or refractive index. The optical data communication system's performance (e.g., bandwidth-density and power-density) depends on the rate at which data is encoded and the energy dissipated during the encoding of each data symbol (e.g., each bit).
Optical data communication technologies that propagate optical signals through a silicon medium are known as silicon photonic systems. In a silicon photonic system, the plasma dispersion effect may be used to control the concentration of free charge carriers in a semiconductor device, thereby modulating the light carried by a nearby optical waveguide. The concentration of free-carriers may be controlled by carrier injection, carrier depletion, or carrier accumulation techniques. For example, injection-based modulation may be performed using a PIN diode with a silicon waveguide embedded in its intrinsic region. Forward-biasing the PIN diode causes carriers to be injected into the intrinsic region, thereby changing the silicon waveguide's refractive index. As another example, depletion-based modulation may be performed using a PN junction diode with a silicon waveguide embedded in the PN junction region. Reverse-biasing the PN junction causes carriers to be removed from the PN junction region, thereby changing the waveguide's refractive index. As yet another example, accumulation-based modulation may be performed using a device with an insulating layer between P and N regions of a diode (e.g., a MOS capacitor).
Although silicon's refractive index is only weakly dependent on the concentration of free charge carriers, a ring resonator structure for enhancing this dependence has been proposed (Lipson, Nature 2004, p. 1082). Use of such a ring resonator structure may facilitate low-power optical modulation in silicon.